Due to a wide range of potential applications and new revenue opportunities, M2M communication is arousing increasing interest in recent years, particularly among mobile network operators, vendors, as well as research entities. It is expected that the number of M2M devices connecting to mobile networks will increase significantly in the next couple of years. According to recent statistics, a total of 50 billion connected devices are predicted for the year 2020.
However, the current mobile networks are designed and optimized only for traffic characteristics of human-based communication applications, while M2M communication has substantially different features. For instance, M2M communication is uplink-dominant and employs small-packet transmission with a low data rate. Therefore, the predicted rapid deployment of M2M communication will pose significant challenges to the current mobile networks, in particular when accommodating both human-based and machine-based services of a wide range of characteristics and requirements.
In particular, many types of networks, including said mobile networks, often observe bursty traffic from machine-based communications. Bursty traffic means that many M2M devices are triggered simultaneously, and that a bulk of data traffic is consequently generated in the uplink direction. Such synchronized behavior from a multitude of M2M devices may, for example, be caused by an external event, which triggers the massive number of M2M devices to at once, i.e. simultaneously, request for a connection to a central unit of the network, for example to a base station. As an example, a large number of metering devices may become active almost simultaneously after a period of power outage. As another example, synchronized behavior may be caused by many M2M devices in a mobile network being simultaneously handed over from one base station of the mobile network to another. The simultaneous handover may be caused by a highly synchronized mobility of the M2M devices or by the outage of a base station.
In the state of the art, for requesting a connection of a device to the central unit typically the RA procedure is employed. When a device is triggered by an event, and when the device has information to report to the central unit, the device will initiate the RA procedure to request initial access to the central unit. The RA procedure comprises in particular four steps, which are shown in FIG. 1.
In a first step, the device transmits its identity and the cause of the access request using a randomly selected RA preamble sequence on a Physical Random Access Channel (PRACH) to the central unit, which is in this case a base station. In a second step, the base station detects the preamble sequence, and transmits a RA response on a Physical Downlink Shared Channel (PDSCH) responsive to the detected preamble sequence. In a third step, the device transmits other messages, e.g. a scheduling request, to the base station using Physical Uplink Shared Channel (PUSCH) resources, which are assigned by the base station to the device in the RA response transmitted in the second step. In a fourth step, the base station confirms the messages received in the third step on the PDSCH.
A problem of the above described RA procedure is the limited amount of RA resources, which are typically preamble sequences. For example, in LTE-Advanced and LTE, each cell of a mobile network is assigned only 64 preamble sequences. Even more severe, among those 64 preamble sequences, only a limited number can be utilized for the RA procedure. Therefore, the probability of a collision is high, in particular when many devices initiate a RA procedure at the same time. A collision will occur, if two or more devices have selected the same preamble sequence.
In case M2M communication is used in a network, the number of devices, which simultaneously attempt to access the central unit of the network, wherein the devices may include both User Equipments (UEs), i.e. human-based devices, and M2M devices, can increase sharply, particularly due to bursty traffic of the M2M devices. Bursty traffic of a large number of M2M devices will inevitably lead to an overload of the central unit of the network, and will also increase the probability of collisions in the above-described first step of the RA procedure. As a consequence, a severe signaling congestion can occur in the PRACH.
The messages, which are the cause of and have to be dealt with during a signaling congestion, can be classified into two categories. The first category concerns redundant messages, i.e. messages containing the same relevant information, which are sent repeatedly by different devices. In other words, the transmission of a message by a certain device is unnecessary, if the same message or a message containing the same content has already been transmitted to the central unit by one or more of the other devices. An example for the occurrence of such redundant messages is multiple handover requests, which are sent by a group of tracking tags provided in the same vehicle. The second category concerns diverse messages, i.e. different messages transmitted synchronously. An example for such diverse messages is the transmission of diverse impact sensors in case of a car crash. Another example is a high number of metering devices becoming active almost simultaneously after a period of power outage.
If, on the one hand side, redundant messages are not properly handled in the case of a signaling congestion, the devices may initiate requests for sending messages with the same or a different central unit of the network at the next time slot, or even over and over again each time slot. Thereby, the signaling congestion is even worsened, which should necessarily be avoided. If, on the other hand side, many devices try to transmit diverse messages, the central unit is significantly challenged due to a limited amount of PUSCH resources, the amount being insufficient for the large number of received transmission requests.
In 3GPP TR 37.868 V11.0.0, “Study on RAN Improvements for Machine-Type Communications”, September 2011, the state of the art proposes six solutions for improving the RA procedure, particularly for dealing with the above-described signaling congestion and with an overload of a central unit of a network in case of M2M communications.
The first solution is the Back off Scheme, wherein the RA procedure of some M2M devices is delayed for a certain number of time slots.
The second solution is the Slotted Access Scheme, wherein each M2M device is only allowed to transmit the RA preamble sequence at specific slots within specific radio frames.
The third solution is the Access Class Barring (ACB) Scheme, wherein a base station can bar or delay a M2M device from reinitiating its RA procedure.
The fourth solution is RACH Resource Separation, wherein human-based and machine-based devices are allocated orthogonal RACH resources.
The fifth solution is the Pull-based Scheme, wherein the M2M devices initiate the RA procedure upon receiving a paging signal from a base station, which is triggered by a M2M server.
The sixth solution is Dynamic Allocation of RACH Resources, wherein the base station dynamically allocates additional RACH resources based on a RACH load condition and on the overall traffic load.
However, the general idea of all these solutions is to instruct the M2M devices with a proper back off time, in order that the M2M devices reinitiate their connection or attach request to the central unit using proper RA resources. None of the proposed solutions properly addresses the major cause for bursty M2M traffic, namely a synchronized behavior of a large number of highly correlated M2M devices. For instance, a plurality of devices in proximity to each other may all be triggered by a common event, for example a tsunami, an earthquake, a blaze or the like. A plurality of M2M devices may also become active after a period of power outage.